Langerhans/dendritic cells (DC) appear to be critical in the establishment of mucosally acquired HIV infection. Primary HIV infection is largely initiated by macrophage tropic (M-tropic) viruses which use the beta-chemokine receptor CCR5, in addition to CD4, for entry. This occurs despite concomitant exposure to T-cell tropic (T- tropic) viruses which use the beta-chemokine receptor CXCR4 in addition to CD4 for entry. The mechanisms of this restriction are unknown. Individuals homozygous for a 32 base-pair deletion in CCR5 (delta- 32CCR5) lack cell surface expression of CCR5 and are with rare exception protected from HIV infection although they have normal CXCR4 expression and their cells are infectable with T-tropic viruses in vitro. The purpose of these studies is to evaluate these delta-32CCR5 homozygous individuals to determine whether DC contribute to the M- tropic restriction of primary infection. We pulsed DC from wild-type (normal CCR5 genotype, WT) and delta-32CCR5 individuals with limiting dilutions of HIV strains of different tropisms and co-cultured them with autologous or allogeneic CD4+ T cells; culture supernatants were monitored for productive infection by RT assay. We also stimulated CD4+ T cells with PHA and infected them with limiting dilutions of the same HIV strains, monitoring for infection by RT assay. We found that T- tropic, but not M-tropic viruses enter delta-32CCR5 DC, whereas both types of viruses efficiently enter WT DC. However, despite the lack of viral entry, DC from delta-32CCR5 individuals are capable of transmitting M-tropic virus to WT CD4+ T cells; this occurs even when CD4+ T cells are added to DC up to 72 hours after pulsing with HIV. The time required for DC to migrate from the mucosa to draining lymph nodes is approximately 24-48 hours. Thus, the block to infection with M- tropic viruses in delta-32CCR5 individuals would seem to be at the level of the CD4+ T-cell, not the DC. T-tropic HIV strains are passed from both WT and delta-32CCR5 DC to autologous or allogeneic CD4+ T cells with similar efficiencies. Thus, in this system of DC-CD4+ T-cell interaction, DC do not seem to be responsible for the protection from T-tropic viruses observed in vivo in delta-32CCR5 homozygous individuals. - HIV; Dendritic cells; CCR5; M-tropic; T-tropic; beta- chemokines; T-cells - Human Subjects